After making the point that justification comes by the faithfulness of Christ, Paul then counters a claim by the Judaizers that said, “Well, now the law is gone I guess you Christians can do what you want?” (v.17). Paul’s response is emphatic: absolutely not. He goes on to say (in the first person but most likely aimed at Peter) that if I build up again those things I once destroyed, I demonstrate that I am one who breaks God’s law. Simply, Peter retreating into Jewish rule-following and perceived outward signs of justification shows that he is a law follower and therefore a law breaker because that is what law does: shows us where we’ve broken it.
As it was law that killed Jesus (Luke 22.66-23.25) Paul writes that through the law I died to the law so that I may live to God. Jesus died but Paul writes that “I” died…? The law demands death, Jesus paid that price, and through his union with Christ by faith (Romans 6.1-6, 1 Corinthians 12.13) Paul can now say first person, that it is no longer I who live, but Christ lives in me. This union is only possible through faith in the faithfulness of Jesus who loved us and gave himself for us. Paul underscores this point and writes that if all of this could have happened through the law – ritualistic religion and ascetic living, as modelled by Peter in his backsliding – then Christ died for nothing.
For you and for me the point is that by faith in the faithfulness of the Son of God (v.20) we too can live a life that is dead to the law and its effects on us because of the truth that the Son of God gave himself for us. If it were possible for you to justify yourself before God then, as Paul writes, Christ died for nothing. This would have been the most monumental and catastrophic mistake and waste of time in history. But you can’t. You can’t justify yourself through works of law (v.21), so, consequently, Christ didn’t die for nothing. He died on that cross to defeat law, sin, and death’s power over you so that you can live because of the faithfulness of the Son of God. Consider yourself dead to law, sin, and death’s power over you, and consider yourself alive in Christ.